Nissan Repair: Smoke from exhaust., valve stem seals, nissan maxima

Question
I have a 98 Nissan Maxima automatic 3.0 V6 73000 miles. Recently I started seeing white or bluish white smoke come out of my exhaust. First of all "it only happens" when the car has been driven for like 30 minutes or so. It also does it on start up but only if it smoked the last time i drove. It isn't constant and the car has never overheated. It drives smoothly other than when it smokes, it kinda hesitates. It smokes a whole lot(big cloud that is really thick) when I make turns from a stop and dissipates after about a minute. And it sometimes smokes a little (few little puffs as it changes gears) as I accellerate from a stop. I have brought it to a shop and they did a compression test and checked the plugs, both came out good.
He said that it isnt the head gasket and it probably is the valve stem seals. So i brought it to someone and they replaced seals. When i got it back I went driving and it did it again, maybe not as much (only once) but I didnt drive that long. After that I also replaced the pcv valve. And took it back out and it did it a little again (only once). Since the repairs, I have driven many times running errands and never see any smoke while driving or at all when i start it. Ok now I have a few questions.

First, will it keep smoking for a while after the seals are replaced due to oil that leaked before, if so how long?

Are the valve stem seals the same as the valve seals?

If it isn't any of these problems what else could it be?

Thanks for the help.

Answer
OK Cayde.  You have blocked oil return passages in the heads.  This is where the oil drains back into the pan after being pumped up to the head.  You can change to a synthetic oil like Mobil 1 5W30 and see if that melts the old oil out and gets it back into solution or you can take the valve covers off and clean out the return passages.  The valve cover removal is the sure way to get the job done.  But, if it is crusty you have to be careful not to let the crispy stuff fall down into the engine.  I like to use an old vacuum and hose to remove those bits.  Then reinstall the valve covers and change the oil.  You should be good to go.